I have the original hoody and jacket, and they fit me normal as per the sizing. I wear large in pretty much everything, and these are right in there as normal for how a large fits me.
They both have space to comfortably do a base or two but not much more. They are not supposed to be more than just a base under, and i do not think i have done anything more than a single base layer.
So, depending on how you want to use this, you could go either way i think. The one caveat is if you have long arms. The arm length on these is cuffed on the “short normal” side. They are fine for me but i could use another half inch on both and be fine. If you have long arms, then i would recommend the larger size.